Based on the mechanisms of action of toxicity of CO and HCN and the supporting literature, it is likely that the toxicities of these two chemicals are additive, and therefore, the hazard presented from combined exposures to these chemicals should be assessed as a mixture and not singularly or individually.
The use of the HQ approach proposed by the Army is reasonable in establishing exposure limits for personnel simultaneously exposed to CO and HCN.
CO is assessed as an individual chemical in HHAs using the Coburn-Forster-Kane (CFK) equation for predicting the percent of COHb in blood. The use of the CFK model for the prediction of COHb levels related to air concentrations of CO is justified. The CFK model has been validated; however it has not been tested in environments with dynamically changing air concentrations, such as in an armored vehicle.
The use of an air concentration for HCN in the HQ equation, as opposed to a blood level, is reasonable.
The Army should conduct further neurological studies on sensory and motor performance at lower concentrations of HCN and CO because most studies on the combined toxicity of CO and HCN have been carried out at high concentrations and have focused on lethality and/or incapacitation; this makes it difficult to use those data to extrapolate to low-levels of exposures and more subtle toxicity end points of interest to the Army. The committee recommends that the Army assess the validity of the CFK model in the context of armored vehicles both using instantaneous measured data and various running averages.
While the toxicity of combined exposures to HCN and CO is important to understand, the Army should also consider concurrent exposures to other chemicals, e.g., other combustion gases, diesel exhaust, which may have additional effects on the tank crew.